The present invention is directed generally to coupling light from one location to another, and more particularly to a resonant cavity enhanced waveguide grating coupler.
Coupling of light from one point to another or one location to another, particularly in optical transmission of information, production of high power coherent light, pump sources for laser, and the like. The challenges presented are illustrated, for example, in the context of developing high power vertical cavity surface emitting lasers (VCSELs).
High-power VCSEL arrays have been demonstrated by several research groups. Grabherr et al. reported VCSEL power densities exceeding 300 W/cm2 from a 23-element array [M. Grabherr et. al., Electron. Lett., vol. 34, p.1227, 1998]. Francis et al. demonstrated VCSEL power in excess of 2-W continuous wave and 5 W pulsed from a 1000-element VCSEL array [D. Francis, et. al., IEEE Int. Semiconductor Laser Conf. (ISLC), Nara, Japan, October 1998]. Chen et al. also reported the power density of about 10 kW/cm2 Steradian from an array of 1600 VCSELs using a microlens array to individually collimate light from each laser [H. Chen, et. al., IEEE Photon. Technol. Lett., vol. 11, No. 5, p. 506, May 1999]. However, their beam quality at high power is still poor. A high quality beam requires a narrow linewidth single mode with high spatial and temporal coherence.
In order to produce coherent, single-frequency, high-power arrays of VCSELs, the elements of one or two dimensional VCSEL arrays should be phase-locked. Although the light from each individual VCSEL is coherent, the phase and frequencies (or wavelengths) of the light from each VCSEL are slightly different and therefore uncorrelated. For such an incoherent array comprising N elements producing the same power P, the on-axis power in the far-field is xcx9cNP. However, if the array as a whole can be made coherent, in phase, and with a single frequency, the on-axis power in the far-field is N2P and the width of the radiation pattern is reduced by xcx9c1/N. Previous efforts to phase-lock arrays of VCSELs have used diffraction coupling [J. R. Legar, et. al., Appl. Phys. Lett., vol. 52, p.1771, 1988], and evanescent coupling.[H. J. Yoo, et. al., Appl. Phys. Lett., vol. 56, p.1198, 1990]. Diffraction coupling depends on geometrical scattering of light and evanescent coupling requires that the optical field of adjacent array elements overlap. Both approaches impose restrictions on the array architecture. More importantly, these existing approaches have had very limited success, even in 1D edge-emitting arrays where both approaches have been extensively investigated. Recently, Choquette et al. has demonstrated phase locking in a VCSEL array using an antiguide approach [D. K. Serkland, et. al., IEEE LEOS Summer Topical Meeting, p.267, 1999].
The above problems and disadvantages of previous attempts to provide effective and efficient coupling of light are overcome by the present invention which includes a waveguide and surface normal gratings.
In accordance with the present invention, an optical coupler is provided comprising first and second mirrors, wherein the first mirror is positioned with respect to the second mirror so that a resonant cavity is defined between them, a waveguide structure positioned in the resonant cavity and including a surface-normal grating structure, and wherein a thickness of the resonant cavity is selected so that a phase matching condition is satisfied for resonance in the resonant cavity.
In one embodiment of the present invention, at least one of the first and second mirrors is formed from a structure in an optoelectronic device. Alternatively, at least one of the first and second mirrors is formed from a semiconductor layer.
In another embodiment, at least one of the first and second mirrors is formed as a semiconductor distributed bragg reflector. Alternatively, at least one of the first and second mirrors is formed as a dielectric distributed bragg reflector.
At least one of the first and second mirrors can be a mirror in a vertical cavity surface emitting laser (VCSEL) structure. Also, at least one of the first and second mirrors is a mirror in a photodetector structure, in a different embodiment.
The first and second mirrors and waveguide structure can be formed in a hybrid structure, or a monolithic structure.
The surface-normal grating structure is shaped so that first order modes of light It incident upon the surface-normal grating structure are coupled into the waveguide while zero-order modes are reflected out into the resonant cavity and reflected by the mirror.
Coupling into and out of a waveguide using a grating is a simple method to transfer free space data to waveguides in optoelectronic integrated circuits (OEICs). Waveguide gratings can perform a large variety of functions such as reflection, filtering, deflection, and input/output coupling. A periodically modulated grating can perform holographic-wavefront conversion. As a coupler, the grating converts a waveguide mode into a radiation mode, or vice versa. Surface-normal grating couplers direct light perpendicularly into and out of the waveguide. The combination of the waveguide and surface normal grating couplers permits arrays of VCSELs to be used to create an optical source with high right angle mode output power. Arrays of VCSELs can then be used as high-power light sources for use in a variety of military and commercial applications, such as free-space optical communications, on-chip/chip-to-chip communications, RF photonics, laser radar, and optical pump sources for solid-state and fiber lasers.
As will be appreciated upon consideration of the following detained description of the invention and accompanying drawings, there are many advantages and features of the present invention, which in turn lead to many new and useful applications of the invention.